The invention relates to a method of exfoliating clay into polyolefins. In particular, the invention relates to treating smectite clay with a Ziegler-Natta catalyst and polymerizing an olefin in the presence of an organoaluminum cocatalyst and the treated clay.
Polyolefins are widely used because of their properties. Nevertheless, the applications for polyolefins could be extended if certain properties such as stiffness, strength and heat resistance were improved. While fillers can improve these properties, their use is limited because there does not exist a good method for dispersing the fillers and achieving the desired properties without concomitant loss of toughness. This is presumably due to the high levels of fillers needed and concomitant problems with dispersing the fillers in the polyolefin matrix. There is a need for an improved method to disperse clay filler into a polyolefin matrix.
U.S. Pat. Nos. 5,830,820; 5,906,955; 5,925,587; 6,034,187 and 6,110,858 provide supported catalysts for the polymerization of olefins. Low levels of these supported catalysts are then used to catalyze the polymerization of olefins and provide polyolefins with only low levels of the support material.
U.S. Pat. No. 6,252,020 provides for clay-filled compositions by bulk and suspension polymerization of vinyl monomers such as styrene in the presence of clay and catalysts such as peroxides. Neither the polymerization of olefins such as ethylene or propylene nor the use of transition metals as catalysts is described or suggested.
U.S. Pat. No. 4,473,672 describes a process for making polyolefin compositions with a variety of fillers such as graphite, carbon black, an aluminosilicate clay, mica, talc, vermiculite or glass fibers by pretreating the filler with an organic magnesium compound and then adding the resultant composition to a transition metal and subsequently initiating the polymerization with an organoaluminum compound.
U.S. Pat. No. 4,564,647 teaches a process for producing a filled polyethylene composition with a variety of fillers. The process is general with regard to fillers. Specifically mentioned are metals, metal oxides, metal carbonates, titanium dioxide, mica, glass beads, glass fibers, silica, alumina, silica aluminate and organic pigments among many others. The filler may take various forms, such as powder, granule, flake, foil, fiber and whisker. The catalyst component is a transition metal treated with either a magnesium or manganese compound or is a Group 4 cyclopentadienyl compound. Despite a very broad disclosure, there is no mention of clay and no indication of a method of exfoliating clay.
PCT Int. Appl. WO 01/30864 discloses a method for producing a nanocomposite polymer by use of an acid-treated, cation-exchanging layered silicate material. The reference teaches that the silicate material is acidified by contacting it with a Bronsted acid such as a mineral acid or an amine hydrochloride. This requires an extra step, which increases the cost and complexity of the process. We found that the acid can also a have deleterious effect on the yield of the polymerization process, particularly when a Ziegler-Natta catalyst is used instead of a metallocene complex.
It has been observed that the synthesis of polyolefin-silicate nanocomposites remains a synthetic challenge (Bergman et al., Chem. Commun. (1999) 2179). These workers attributed the difficulty to the sensitivity of the vast majority of olefin polymerization catalysts to Lewis bases and water. Therefore, they used late transition metal catalysts to attempt to polymerize ethylene in the presence of a synthetic fluorohectorite. The product formed was described as a rubbery polymer that was highly branched. Such a polymer is unsuitable for many applications because of difficulties in processing.
There is a need for a simple process for providing clay-filled compositions and, in particular, for polyolefin compositions containing exfoliated clay.
The invention is a process for incorporating clay into polyolefins. The process involves treating smectite clay with a hydrocarbon solution of a Ziegler-Natta catalyst and polymerizing the olefin in the presence of the treated clay and an organoaluminum cocatalyst.
This invention provides for a simple method to prepare polyolefin compositions that contain exfoliated clay platelets. The invention also includes clay-filled polyolefin compositions prepared by this method.
The clays useful in the invention are non-acid-treated smectite clays. Smectite clays are well described in the literature (see Izumi, Y. et al., Zeolite, Clay and Heteropoly Acid in Organic Reactions, VCH Publishers Inc. (1992)). They are layered materials with exchangeable cations between the layers to compensate for the negative charge of the layers. Clays are classified according to their layer charge. Smectite clay minerals have cation exchange capacity in the range of 60-100 meq/100 g-clay.
Smectite clays can be synthesized from magnesium silicates. Synthetic smectite clays are available from ZEN-NOH UNICO America Corporation. More commonly, they are available from naturally occurring bentonite ore. Two common types of smectite clay are montmorillonite and hectorite. Montmorillonite is classified as magnesium aluminum silicate and hectorite as magnesium silicate. Montmorillonite is more available due to the vast naturally occurring deposits.
By xe2x80x9cnon-acid-treated,xe2x80x9d we mean that the clay has not been treated with a Bronsted acid to exchange the cations with a proton. Bronsted acids are acids that can donate a proton. Examples include HCl, H2SO4, triethylammoniumchloride and N,N-diethylanilinium chloride.
The cations on the clay surface affect the organophilicity of the clay. If the cation is a metallic cation such as sodium or calcium, the clay is not very organophilic and will not dissolve in organic solvents such as toluene. These clays are useful in the invention. However, optionally, it may be preferred to use a more organophilic clay. If the cation is an organic cation such as an ammonium cation, then the clay becomes more organophilic. These are readily prepared by cation exchange of the sodium clay with an organic cation. Suitable organic cations include ammonium cations where the nitrogen has four non-hydrogen substituents, such as hexadecyloctadecyldimethyl ammonium, dimethyldioctadecyl ammonium, benzyl triethyl ammonium, methyltrioctylammonium and poly(oxypropylene)methyldiethyl ammonium. This increases the solubility and ease of dispersion in organic solvents. Dependent upon the amount of cation exchange and the particular organic cation used, the clay may be soluble in organic solvents such as toluene.
Optionally, the clay can be surface treated to react hydroxyl groups on the clay and to increase the organophilicity of the clay. By reacting the hydroxyl groups on the clay, the catalyst performance and hydrogen response is often improved. By xe2x80x9chydrogen response,xe2x80x9d we mean the ability to incorporate hydrogen as a means of controlling polyolefin molecular weight. The surface treatment can be done with a silicon compound or with a monoalkyl metal compound. Preferably, the surface treatment is done with a silicon compound and preferably the silicon compound is an alkyl disilazane. Suitable alkyl disilazanes include hexaalkyl disilazanes having the formula R13SiNHSiR13 where R1 is a C1-C20 hydrocarbyl. In particular, hexamethyldisilazane is preferred. Preferred monoalkyl metal compounds contain a single C1 to C8 alkyl group, as in ethyl aluminum dichloride, isobutyl aluminum dichloride or methyl magnesium chloride.
Optionally, the clay is dried. When the clay has an organic cation or has been treated with an organosilicon compound, it is less hydrophilic and has a tendency to retain less water. For these clays, the drying step is less important. When the clay has a metal cation, it is more hydrophilic and therefore it is preferable to dry the clay. If the clay has a metal cation, preferably the drying is done at a temperature of from about 50xc2x0 C. to about 600xc2x0 C., more preferably from about 100xc2x0 C. to about 400xc2x0 C. If the clay has an organic cation or has been treated with an organosilicon compound, preferably the drying is done at a temperature of from about 50xc2x0 C. to about 250xc2x0 C., more preferably from about 50xc2x0 C. to about 150xc2x0 C. All clays are preferably dried with vacuum or with a stream of dry nitrogen.
The clay is treated with a Ziegler-Natta catalyst. By xe2x80x9cZiegler-Natta catalyst,xe2x80x9d we mean a transition metal compound that incorporates a Group 4-8 transition metal, preferably a Group 4-6 transition metal, and one or more ligands that satisfy the valence of the metal. The ligands are preferably halide, alkoxy, hydroxy, oxo, alkyl, and combinations thereof. Preferred Ziegler-Natta catalysts incorporate Ti, V, or Cr, most preferably Ti. Preferred Ziegler-Natta catalysts also have high thermal stability. They include titanium halides, titanium alkoxides, vanadium halides, and mixtures thereof, especially, TiCl3, TiCl4, mixtures of VOCl3 with TiCl4, and mixtures of VCl4 with TiCl4. Suitable Ziegler-Natta catalysts also include the transition metal compound admixed with various metal halides such as TiCl3 with magnesium chloride or mixtures of VCl4 and TiCl4 with aluminum chloride. Other suitable Ziegler-Natta catalysts appear in U.S. Pat. No. 4,483,938, the teachings of which are incorporated herein by reference, and in Eur. Pat. 222,504.
The catalyst is dispersed, dissolved, or suspended in a compatible organic solvent such as heptane or toluene and added to the clay. The amount of organic solvent can be chosen so that the catalyst solution is just enough to wet the surface of the clay or more solvent can be used to create a slurry or solution of the clay. Optionally, the clay can be predispersed in the organic solvent and then the catalyst or a catalyst solution added. Dependent upon the organophilicity of the clay and the particular solvent chosen, the clay may appear as a damp solid or if sufficiently organophilic may appear to dissolve in the solvent.
When the clay is insoluble, the clay is preferably mixed to ensure good distribution. A convenient way of mixing is to put the treated clay in a bottle on a roll mill.
When a monoalkyl metal is used as a surface treatment, it is preferred to thoroughly mix the clay in an organic solvent prior to adding the monoalkyl metal and the catalyst. This can be done by stirring the clay in the solvent prior to the addition of the monoalkyl metal and the catalyst. The period of time necessary for thorough mixing will vary based upon the shear rate of the stirring.
The treated clay may be used as is with solvent present or optionally the solvent may be removed. If the clay is dissolved in the organic solvent, it is preferable to use the solution as is in the subsequent polymerization. If the clay is insoluble, it is preferable to remove the solvent with vacuum to form a more easily handled solid.
The organoaluminum cocatalyst is an alkyl aluminum or an alkyl aluminum halide. Preferred alkyl aluminums include trialkyl or triaryl aluminum compounds, which preferably have the formula AlR5R6R7 where R5, R6 and R7 denote the same or different C1-C20 hydrocarbyl. Particularly preferred alkyl aluminums are trimethylaluminum, triethylaluminum, tripropylaluminum, and triisobutylaluminum. Suitable alkyl aluminum halides include dialkyl aluminum halide and alkyl aluminum dihalide compounds, which preferably have the formula AlR5R6X or AlR5X2 where X is Cl, Br, or I.
Exemplary alkyl aluminum halides are dimethylaluminum chloride, methylaluminum dichloride, diethylaluminum chloride, ethylaluminum dichloride, diisobutylaluminum chloride, isobutylaluminum dichloride, methylaluminum sesquichloride, ethylaluminum sesquichloride, and isobutylaluminum sesquichloride.
Optionally, silicon compounds may be used in the polymerization. These can offer certain improvements such as an improved sensitivity to hydrogen as a means of controlling molecular weight. Preferred silicon compounds are dialkyl dialkoxysilanes which have the formula R1R2Si(OR3)(OR4) where R1, R2, R3, and R4 denote the same or different C1-C20 hydrocarbyl. Exemplary dialkyl dialkoxysilanes are cyclohexylmethyldimethoxysilane, dimethyldimethoxysilane, dimethyldiethoxysilane and dicyclopentyldimethoxysilane.
Suitable olefins for the polymerization are C2-C20 xcex1-olefins, such as ethylene, propylene, 1-butene, 1-hexene, 1-octene and mixtures thereof. Preferred olefins are ethylene, propylene and mixtures thereof with xcex1-olefins such as 1-butene, 1-hexene and 1-octene.
The treated clays can be used in a variety of well-known olefin-polymerization processes, including gas, high pressure liquid, slurry, solution, or suspension-phase techniques and combinations of these. The pressures used typically range from about 15 psig to about 15,000 psig. Polymerization temperatures range from about xe2x88x92100xc2x0 C. to about 300xc2x0 C., more preferably from about 20xc2x0 C. to about 200xc2x0 C., and most preferably from about 60xc2x0 C. to about 150xc2x0 C.
The clay imparts improved properties such as stiffness and barrier properties including a decreased rate of moisture vapor transmission. In the process of the invention, the clay becomes exfoliated, thereby improving the dispersion of the clay and enabling the improved properties without severe loss of other properties such as impact or toughness. Smectite clay has a multilayer structure. By xe2x80x9cexfoliation,xe2x80x9d we mean breaking the layered structure to improve the dispersion of the clay in the polyolefin. By analogy with a deck of playing cards, non-exfoliated playing cards would be present as groups of 52 stacked playing cards, while exfoliated playing cards would be more dispersed and principally present in groups of substantially fewer than 52 with some cards even being dispersed as single cards. The greater the exfoliation, the better the dispersion and the more effective a certain level of clay at improving the desired properties.
Dependent upon the application, the level of clay in the polymer can be varied. Preferably, the clay will be present at about 0.1% to about 15% by weight. More preferably, the clay will be present at about 1% to about 10% by weight, and most preferably at about 4% to about 6% by weight.
The following examples merely illustrate the invention. Those skilled in the art will recognize many variations that are within the spirit of the invention and scope of the claims.